Disposal of spent toners has long been a major problem for users of electrostatic plotters, printers and copiers. Environmental awareness, disposal costs, and strict governmental regulations relating to chemical handling and disposal have threatened the present manner of use of liquid toners. Alternatives that reduce disposal amounts and allow for reuse of some of the materials are clearly needed.
A method already exists, in principle, for eliminating the need for liquid toner disposal. This is described in U.S. Pat. Nos. 4,799,452; 4,895,103 and 4,923,581, all to G. F. Day. In this method, the toner itself is eliminated except for a transitory existence just at the moment of toning. Concentrated "ink" of each color is stored in a small tank and injected into and mixed with a continuous stream of clear dispersant. The resulting toner stream is passed through the toner applicator and then quickly decomposed back into concentrate and dispersant. This is done electrophoretically with a separator, described in the above-referenced patents. The solid pigment particles are plated out on a rotating drum, then scraped off the drum and re-dispersed by vigorous mixing into the concentrate holding tank. To stop the toning process the injection of the selected concentrate is simply terminated.
It would seem that this recycling concept might provide a liquid toning technology free of disposal problems since a large volume of contaminated or spent toner would never exist. However, the basic cause of disposal is not eliminated. Eventually the contents of the concentrate tanks would have to be discarded due to contamination as would the fluid in the dispersant tank. This is because the contaminants are re-mixed with the dispersant along with the pigment particles and are, therefore, never removed from the system. The quantity of liquid to be thrown away would be smaller, but some disposal problems would remain. The dispersant would have to be discarded when the conductivity level became high enough to interfere with image toning. A much higher level of contamination could be tolerated in the color concentrate tanks because of the dilution upon injection into the dispersant stream, but eventually the concentrate would also need replacement. In addition, the recycling architecture is relatively complex. It requires precise metering and mixing of two fluid streams and high speed separation of the toner into its components as it flows out of its applicator. With the high flows which are characteristic of full-width toner applicators, the separation apparatus must be quite large and, therefore, costly.
In order to electrophoretically separate a toner stream into its components, the fluid is passed between two closely spaced, parallel electrodes while a high voltage is imposed across the gap. All of the fluid must be exposed to the full electric field and this means the flow must normally be confined to the gap region with some kind of fluid seals along the lateral edges of the separation zone. One of the electrodes must also be moving so that the accumulating sludge can be scraped off and sent to the appropriate concentrate tank. The seals which confine the fluid flow within the gap present numerous technical difficulties. They must support the high voltage and this high voltage appears across the exposed surface of the seal which joins the two electrodes. Surface electrical breakdown keeps the applied voltage lower than would otherwise be desired and this, in turn, causes the separator to be yet larger in order to assure total separation of the solids from the clear fluid. Also, depending on the specific toner formulation, redispersion of the deposited solids layer into the color concentrate tank can be difficult and energy intensive. Thus, the range of toner formulations which can be employed for this purpose is limited.
Commonly assigned U.S. patent application Ser. No. 08/204,884 describes a method of continuously purifying small portions of the liquid toner by using a rotating drum type of electrophoretic separator to keep contamination below the level at which it will interfere with imaging. That method would benefit from a higher level of purification for each pass, however. Rotating drum-type separators are effective at removing solid contaminants such as particles of color pigment, paper debris, and ions from the fluid dispersant. In a typical spent liquid toner, however, neutral toner molecules which ionize slowly are also present. These neutral molecules are relatively unaffected by a drum-type separator. After they pass through the drum separator, they largely remain in the fluid where their slow and continuing ionization causes an undesirable increase in conductivity that interferes with electrostatic imaging. Thus, the neutral molecules act as a source for new ions. Removal of these "ionizable" molecules is necessary for complete purification of the liquid. This requires application of an electric field for a sufficiently lengthy period so that the neutral molecule source is itself depleted.
Spiral-type separators have also been used for purifying fluids and are known from the prior art. For instance, U.S. Pat. No. 5,192,432 to Andelman describes a flow-through capacitor for use in a chromatography system. The capacitor contains a plurality of spirally-wound, spaced-apart layers. Fluid flows through porous layers and is subject to an electric field across conductive layers. A purified fluid then flows from the separator. Spirally-wound separators have also been used to decontaminate diesel fuel. See U.S. Pat. Nos. 5,149,433 and 4,620,917.
Difficulties arise in the use of spiral separators with spent toners, however. The spiral separator can remove all contaminants including toner solids, paper debris, ions, and ionizable molecules. Generally, though, the spiral separator will stop functioning when a layer of solid deposits forms on one of its conductive layers and interferes with the electric field. The solid deposits which accumulate on the conductive layers tend to be more insulating than the liquid and this causes the applied voltage to appear entirely across the deposited layer, leaving zero electric field in the liquid. For this reason, spiral separators need to be replaced on a regular basis. This short operating life leads to the problem of disposal or recycling of the separator itself, as well as the high cost of supplying new separators. Similar limitations are to be expected when purifying any other liquid of a variety of contaminants, particularly when significant amounts of solid contaminants are present.
It is an object of the present invention to reduce disposal amounts of liquids having contaminants and to increase the likelihood of reuse of such materials.
It is another object to provide a purification system which is simple, economical, easy to use and durable.
It is yet another object to provide an electrostatic printing system with improved image quality and improved image consistency.
It is yet another object to provide an electrostatic printing system in which frequent replacement of materials is unnecessary.